Multichannel radio receiving system



Aug. 23, 1958. P. F. G. HOLST ET AL 2,127,815

MULTICHANNEL RADIO -RECEIVING SYSTEM Filed Aug. 29, 1936 o MAW m 30K GGK E@ L WM PM N .Jill Q1. rmmwm o@ SS \Nm\ m Pw m. l KNQ I.

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UNITED Patented Aug. 23, 1938 TES PATENT OFFFIQE Collingswood, N. J.,assignors to Radio Corporatonlof America, a corporation of Delaware iApplication August 29,

l1 Claims.

The present invention relates to radio receiving systems having two ormore receiving channels which may provide a combined output signal asshown in our copending application Serial No. 98,443, led August 29,1936, for I-Iigh fidelity radio receivers, and assigned to the sameassignee as this application.

A receiver to which the present invention may be applied may comprise apair of signal receiving 110 and amplifying channels or circuits, one ofwhich is broadly responsive to received signals and the other of whichis sharply responsive thereto, said channels being further controllableto render the broad channel effective only in response to signals of apredetermined high amplitude whereas the sharp channel is utilized forweaker signals.

In this manner, the fidelity or overall response of the receiving systemimproves with increased carrier wave strength, giving a relatively highdegree of fidelity for local radio programs or in response to relativelystrong modulated carrier waves. 'Ihe signal output from a plurality ofamplifying channels is preferably applied to a common audio frequencyamplifier and loud speaker device for the reason that a singleamplifying channel results in desirable cost reduction and for thefurther reason that the sound from both channels preferably is derivedfrom a common loudspeaker output device without switching connections.

It is therefore an object of the present inven tion to provide animproved audio frequency or second detector and audio frequencyamplifier system for a multiple channel radio signal receiving systemwherein the signals from either channel may be combined in a commonaudio frequency amplifier and applied to a single loud speaker device.

It is a further object of the present invention 4:0 to provide an audiofrequency circuit for combining the signal output from a plurality ofsignal channels in a radio receiving system wherein either or bothsignal amplifying channels may r function to amplify signals, and theoutput therefrom, separately or in combination, may be applied to acommon audio frequency amplifier and loud speaker or other outputdevice, without the intermediary of switching means.

50 It is also an object of the present invention, to

1936, serial No. 98,444

connection with a single audio frequency amplier channel therefor.

The invention will be better understood upon reference to theaccompanyingdrawing` wherein a schematic circuit diagram of a radioreceiving system embodying the invention `is shown in Fig. 1, andconnections for the circuit of Fig. 1 with a high gain amplifier areshown in a similar circuit diagram comprising Fig. y2. The scope of theinventionwill be pointed out in the appended claims.

Referring to the drawing, the figure is a schematic circuit diagram of asuperheterodyne radio receiving system embodying the invention.. Thiscomprises two signal channels operating at radio or intermediatefrequency, or both as shown, with one receiving and amplifying chan nelsharply tuned and the other broadly tuned as indicated. In -asuperheterodyne receiver, each channel may comprise a high or radiofrequency amplifying and tuning unit '5, a first detector 6 and anintermediate frequency amplifier l, connected to the same or separatesignal'sources as indicated at 8 and 9 and tunable to the same ordiffering frequencies.

In the present example, the superheterodyne system indicated, includesalso an oscillator I0 in association with the first detector of eachchannel and the signal channelsfurthermore, preferably are provided withsuitable automatic volume control means II and I2 for controlling thesignal output in accordance with the strength of a received carrierwave, inany suitable well known and understood manner. In the presentsystem, the automatic volume control means may be effective to providesubstantially constant signal amplitude with carrier wave strengthvariations at the output of the intermediate frequency amplifiers,whereby the second detectors are provided with substantially constantsignal strength.

Each of the separate signal amplifying channels is terminated in asecond detector, the sharp channel detector being indicated at I4 andthe broad channel detector being indicated at I5` Both detectors may beof the same type, such as diode rectifier devices, having cathodes Itand anodes II, and are supplied with intermediate frequency signalenergy through suitable coupling transformers I8 preferably of the ironcore fixed-tuned type.

The primary winding I9 of each transformer is connected with the outputcircuit 'ZG of the preceding intermediate frequency amplifier, while thesecondary 2 I, together with a shunt fixed tuning capacitor 22 providesa tuned input circuit for the rectifier in which circuit the highpotential terminal 23 is connected with the anode. The low potentialterminal 24 of the input circuit for the detector I4 is connected to thenegative terminal 25 of a diode output resistor 26, the positiveterminal 2l of which is connected to the cathode I6 and to ground asindicated at 28. Signals received through the diode input circuit fromthe preceding receiving system in the sharp channel are applied to thediode anode II and are rectif-led in the detector I4 and then appear asa direct current potential across the resistor 25 between the positiveterminal 2l and the negative terminal 25, the terminal 25 being negativewith respect to ground or chassis 2B. The audio frequency envelope ormodulation component of the signal also appears across the outputresistor 26 between ground and the terminal 25. A suitable intermediatefrequency by-pass capacitor 36 is provided across the resistor 26 toeliminate therefrom any intermediate frequency currents which may bepresent.

In a similar manner, the broad channel detector I5 is provided with adiode output resistor 3| having the positive terminal thereof 32 con--nected to ground or chassis 33 and to the cathode I6, while the negativeterminal indicated at 34, is connected with the loW potential terminal35 of the tuned diode input circuit.

Signals received through the broad channel and amplified therein, areapplied to the detector I5 and appear as a direct current potentialacross the resistor 3| to establish on the terminal 34 a negativepotential with respect to ground, and variable in accordance with thesignal strength, while the modulation or audio frequency component alsoappears across the resistor BI between ground and the terminal 34.

The diode detector arrangement, therefore, comprises two diode rectifierdevices, one in each channel, each having a cathode connected to chassisor ground and having a diode output resistor connected between groundand the low potential terminal of the tuned input circuit for the dioderectifier.

The terminals 25 and 34 are negative terminals and may be considered asthe output terminals for the two rectier devices and for the two signalchannels from which may be conveyed the audio frequency or modulationcomponent of the received signals.

Each of these terminals is coupled to a common audio frequency amplifiercomprising an input amplifier stage having an amplifier device 3Bcomprising a cathode 39 connected to ground 40, a control grid 4Iconnected to an input lead 42, and an output anode connected with anoutput circuit 46 leading to the remainder of the audio frequencyamplifying system represented by the rectangle enclosure 4l. This isconnected to an output loud speaker device indicated at 48.

The audio frequency output coupling to the circuit 46 includes an outputcoupling reactor 0.9 and iilter resistor 50 provided with a by-passcapacitor 5I for supplying to the anode 45 the operating potential asindicated from any suitable source.

The input lead 42 for the single audio frequency amplifier and theoutput terminals 25 and 35 are interconnected through a resistor networkcom prising a pair of resistors 53 and 54 connected in series betweenthe output terminal of one diode rectifier device and the outputterminal of the other diode rectifier device through output leadsindicated at and 56. The lead 42 is connected to an intermediate tap 51between the two series connected resistors.

Since the cathode 39 of the first audio frequency amplifier 38 isconnected to ground and therefore to the positive terminals of bothdiode rectifier output circuits, the grid 4I is subjected to andreceives a portion of both the biasing and modulation potentials fromboth signal channels, and one or the other of the signal channels maypredominate in supplying signals depending upon the operation of theirrespective automatic volume control means I I and I2, without theintermediary of any switching means. If both channels are operating itwill be seen that the output of each diode is impressed upon the rstamplifier grid 4I.

In order however to keep the inter-channel coupling through the resistoroutput network as low as possible the resistance of the resistor 53should be relatively large or high with respect to the resistance of thediode output resistor 26 and similarly the resistance of the resistorsection 54 should be relatively high with respect to the resistance ofthe diode output resistor 3I. Assuming the resistances of the diodeoutput circuits to be R1 and the resistance of each section of theresistor elements 53 and 54 to be R2 then the ratio B2 Ri should bemaintained as large as practically possible in any receiving circuit.The limit of this ratio is generally determined by the resistance of thediode output circuit which may be 100,000 ohms, for example, and theresistors 53 and 54 may then have a resistance preferably of 2 megohms,for example.

The circuit has the further advantage that either channel may be cut offby simple switching means comprising a two point switch having a switcharm and two contacts 6I and 62. The switching arrangement is effected byconnecting the arm 60 to ground or chassis as indicated at 63, and byconnecting each contact 6I and 62 to the respective negative outputterminals for the diode circuits and in this case with the output leads56 and 55 respectively at the input terminals of the resistors 53 and54.

By connecting the arm 60 to the terminal 6I, for example, it will beseen that the diode output resistor 3I is short-circuited and groundedwhile the input end of the resistor 53 remains connected to the terminal25. If the resistor sections 53 and 54 are equal as is the usual casewhere aqual output is desired from both receiving channels, the gridconnection 42 is then effectively tapped down halfway on the wholeoutput resistor comprising both sections 53 and 54 which is theneffectively connected across the output resistor 26. In this manner theaudio frequency channel receives only the signals from the sharpchannel.

In a similar manner, by connecting the arm 60 to the contact 62 thesharp channel may be cut off and the output from the broad channel maythen be taken through the lead 42 at a voltage tap provided by theterminal 51. The portion of the total output voltage available isdetermined, as before, by the relation between the resistances of thetwo sections 53 and 54 which are preferably equal, although notnecessarily so.

For example if it is desired to receive a signal voltage from the sharpchannel in a greater proportion with respect to that from :thebroadchannel, the tap 51 may be connected electrically more adjacent to theoutput terminal 25 by reducing Athe value of the resistor 53. This maybe done in case that the broad channel is responsive to relativelystrong signals and the same output voltage is desired for both the-broad and sharp channel without adjusting the automatic volume controlmeans for this purpose.

The operation of the system described is as follows: With the switchB,6|-62 open, signals received through the sharp channel and through thebroad channel are rectified in the separate detectors |4 and l5 and boththe audio frequency and direct current components thereof appear betweenground and theA leads..55 and 55 connected with the output terminals 25and 34 respectively. -Since the cathode `39 is connected to ,ground 40and the grid 4| of therst audio frequency amplifier is connected to the`tap 57|, the audio frequency and signal voltages appearing in the leads55 and 56 with respect to ground are applied between the grid 4| and the`cathod 39.

This results in an `increasing signal bias for the grid 4| as the signalstrength increases and the tube 38 may then operate without overloading.The combined signals are applied to the output circuit 46 and appear atthe loud speaker 48. If both signal channels are tuned to the samecarrier wave and the automatic volume control l2 is effective to cut offthe signals from the broad channel until a predetermined higher signalamplitude is obtained, then the sharp channel alone or to a greaterdegree supplies the signals to the loud speaker 43. However as the broadchannel becomeseffective to supply signals the .same are applied throughthe coupling network to the grid 4| without switching change and withoutintercoupling between the two channels, by reason of the relation of theimpedances or resistance of the output resistors 26 and 3| to thecoupling resistors 53 and 54. Any suitable arrangement may be providedfor controlling the two signal channels and any suitable automaticvolume control meansmay be provided for operating the two channels inany suitable relation to eachother, since the mixing of the signals andthe provision of biasing potential for the audio frequency amplifier isautomatically obtained without switching connections. It should beunderstood that the switch 6B is provided merely for cutting ofi` one orthe other of the channels as may be desired, and does not interfere withnormal operation when open.

With this arrangement, for example, the two channels may be turned toseparate signals, such as a broadcast program on the broad channel, forexample, and with a sharp channel tuned to police or S. O. S. signalswhich will be applied automatically to the loud speaker 48 whenreceived, without further attention from the listener. Also, the switch6U may be operated to listen selectively to either channel.

This provides a desirable arrangement in police or aircraft receivers,for example, where the two channels are tuned to differing transmittingstations and both may be heard or either one may be selected by thesimple switching means in the audio frequency mixing circuit. In thiscase, a noise suppresserin each channel is desirable, to cut ofi eitherof the channels when not receiving a carrier wave.

In certain applications it may be desirable to connect the receivingchannels with a high gain audio frequency amplifier tube, as shown inFigure 2, to which attention is now directed and in which the samereference numerals are used for like parts as in Figure 1.

Referring to Figure 2, a. high gain audio frequency amplifier tube ofthe `pentode type, is provided with a control grid 66 connected with amovable contact 61 of a volume control potentiometer 68. The gridcircuit is completed through the potentiometer 68 and a ground orchassis connection 69, and biasing potential is derived from a self-biasresistor 10, as shown. The output anode of the amplifier, indicated at1|, is connected to a suitable output circuit 12 through a couplingcapacitor 13, in conjunction with a coupling resistor 14 in the anodecircuit. Positive B potential for the anode is supplied to the couplingresistor 14 through a filter resistor vl5, as indicated.

The input grid circuit, comprising the resistor element, is coupled tothe output lead 42 from the output terminal 51, between the outputresistors 53 and 451|, through a coupling capacitor 81 which serves totransfer the audio frequency signal to the amplifier grid circuit whileisolating the two circuits against .direct current potential.

With this arrangement, the amplifier tube 65 may be self-biased lorbiased in any suitable manner yindependently of the signal potential,and the potentiometer E'l-GB serves to providev volume control meansdirectly for the audio frequency circuit, following the signal combiningnetwork in connection with the two detectors.

By this means, the signal volume may be adjusted to `any desired levelfor both channels jointly. With the signal channels controlled bysuitable automatic volume control and noise suppression means, thevolume control device 61--68, following the channel mixing network, mayprovide the only volume control means for the receiving system.

While the invention has been described in con.- nection Withasuperheterodynereceiving system, it is `apparent that it may be appliedto any receiving system having two signal receiving channels suitablycontrolledby automatic volume control and/or noise suppression means,with the channels terminating in suitable diode rectifier devices andfollowed `by a single audio frequency amplifier channel.

We claim -as our invention:

1. In a multiple signal .channel radio receiving system, the combinationof a pair of diode signal rectifier devices each having a cathode, arectified signal output terminal and an output impedance connectedvbetween said cathode 4and said terminal, a pair of direct currentconducting output impedance elements connected in series between saidterminals, an audio frequency amplifier device having a cathode andhaving a control grid conductively connected between said impedanceelements, and means interconnecting the `cathodes of said rectifierdevices and the cathode of said amplifier device, whereby signalsapplied to said rectifier devices are rectified and jointly applied withthe direct current component thereof to said control grid.

2. In a multiple signal channel radioxreceiving system, the combinationof a pair of vdiode sig-,nal rectifier devices each having a cathode and`a rectified signal output terminal, a pair of outputresistance'elements connected in series between said terminals, an audiofrequency ampliiier'ldevice having a .cathode `and having a control gridconnected between said resistance elements, and means interconnectingthe cathodes of said rectifier devices and the cathode of said amplifierdevice, whereby signals applied to said rectifier devices are jointlyapplied to said control grid therefrom, said resistance elements havinga relatively high resistance each with respect to the rectifier outputcircuits, whereby said signal channels are effectively decoupled whilebeing interconnected with said control grid.

3. In a radio receiving system having a pair of signal receiving andamplifying channels, the combination of a pair of diode rectifierdevices, one coupled to each of said channels to receive and todemodulate the signal output therefrom, automatic volume control meansfor each of said channels effective to control the amplitude of signalsapplied to said diode rectifier devices, each of said diode rectifierdevices comprising a cathode connected to ground and having a diodeoutput resistor connected at its positive terminal to said cathode andground, a pair of signal and biasing potential output leads, one from anegative terminal of each of said output resistors, a pair of seriesconnected resistor elements each having a relatively high resistancewith respect to each of said diode output resistors connected betweensaid output leads, and a common audio frequency amplifier for saidchannels having an input amplifier tube provided with a cathodeconnected to ground and a control grid connected to an intermediate tappoint between said series connected resistor elements whereby the signaloutput and bias potential from said signal channels are applied jointlyto said amplifier device,

4. In a radio receiving system having a pair of signal receiving andamplifying channels, the combination of a pair of diode rectifierdevices one coupled to each of said channels to receive and todemodulate the signal output therefrom, automatic volume control meansfor each of said channels effective to control the amplitude of signalsapplied to said diode rectifier devices, each of said diode rectifierdevices comprising a cathode connected to ground and having a diodeoutput resistor connected at its positive terminal to said cathode andground, a pair of signal and biasing potential output leads one from anegative terminal of each of said output resistors, a pair of seriesconnected resistor elements each having a relatively high resistancewith respect to each of said diode output resistors connected betweensaid output leads, a common audio frequency amplifier for said channelshaving an input amplifier tube provided with a cathode connected toground and a control grid connected to an intermediate tap point betweensaid series connected resistor elements whereby the signal output andbias potential from said signal channels are applied jointly to saidamplifier device, and means for selectively grounding either of saidoutput leads thereby to render ineffective either of said signalamplifying channels selectively.

5. In a radio receiving system, the combination with means providing twosignal amplifying channels therein, of means for combining the signaloutput from said channels comprising an audio frequency amplifier, adiode rectifier device connected with the signal output end of each ofsaid channels, a diode rectifier output resistor in circuit with each ofsaid diode rectifier de vices, a resistor network connecting said diodeoutput resistors substantially in parallel relation to each other, and acircuit connecting said amplier with said network whereby said amplifiermay receive biasing potential from each of said channels and signalpotentials jointly substantially in parallel.

6. In a radio receiving system, the combination with means providing twosignal amplifying channels therein, of means for combining the signaloutput from said channels, comprising an audio frequency amplifier, adiode rectifier device connected with the signal output end of each ofsaid channels, a diode rectifier output resistor in circuit with each ofsaid diode rectifier devices, a resistor network connecting said diodeoutput resistors substantially in parallel relation to each other, and acircuit connecting said amplifier with said network whereby saidamplifier may receive biasing potential from each of said channels andsignal potentials jointly, substantially in parallel, said resistornetwork providing a relatively high resistance between said channelswhereby they are substantially decoupled while being maintained inparallel connected relation to each other in connection with theamplifier.

7. In a radio receiving system, the combination with means providing twosignal amplifying channels therein, of means for combining the signaloutput from said channels, comprising an audio frequency amplifier, adiode rectifier device connected with the signal output end oi each ofsaid channels, a diode rectifier output resistor in circuit with each ofsaid diode rectifier devices, a resistor network connecting said diodeoutput resistors substantially in parallel relation to each other, acircuit connecting said amplifier with said network whereby saidamplifier may receive signal potentials jointly from said rectifiers,Substantially in parallel, and means in connection with said resistornetwork providing a single variable volume control element interposed incircuit between said amplifier and a potential supply point on saidnetwork.

8. In a radio receiving system, the combination with means providing twosignal amplifying channels therein, of means for combining the signaloutput from said channels, comprising an audio frequency amplifierhaving an amplifier tube, a diode rectifier device connected with thesignal output end of each of said channels, a diode rectifier outputresistor in circuit with each of said diode rectifier devices, aresistor network connecting said diode output resistors in parallelrelation to each other through the negative terminals thereof, a circuitconnecting said amplifier tube with a potential supply terminal of saidnetwork, to receive signal potentials through said network from each ofsaid channels jointly, substantially in parallel, said resistor networkproviding a relatively high resistance between said channels wherebythey are substantially decoupled while being maintained in parallelconnected relation to each other in connection with the amplifier tube,said amplifier tube being of the high mu type, impedance coupling meansincluding a coupling capacitor between said tube and said resistornetwork, and means for applying a biasing potential to said amplifiertube.

9. In a radio signal receiving system having a pair of signal amplifyingchannels, the combination of a diode rectifier device in each channelhaving a cathode and a rectifier output resistor connected to groundwith the cathode, a pair of resistors connected in series between thediode output resistors, switch means for selectively connecting eitherof the outer terminals of said series connected resistors to ground, andan audio frequency amplifier having a cathode connected to ground and acontrol grid connected with a point along said series connectedresistors to receive rectified signals jointly from said amplifyingchannels.

10. In a radio signal receiving system having a pair of signalamplifying channels, the combination of a diode rectifier device in eachchannel having a cathode and a rectifier output resistor connected toground with the cathode, a pair of resistors connected in series betweenthe anode ends of the diode output resistors, switch means forselectively connecting either of the outer terminals of said seriesconnected resistors to ground, and an audio frequency amplifier having acathode connected to ground and a control grid conductively connectedwith a point along said series connected resistors to receive rectifiedsignals and biasing potential jointly from said arnplifying channels.

11. In a radio signal receiving system having a pair of signalamplifying channels, the combination of a diode rectifier device in eachchannel having a cathode and a rectifier output resistor connected toground with the cathode, a pair of resistors connected in series betweenthe anode ends of the diode output resistors, switch means forselectively connecting either of the outer terminals of said seriesconnected resistors to ground, and an audio frequency amplier having acathode connected to ground and a control grid connected with a pointalong said series connected resistors to receive rectified signalsjointly from said amplifying channels, said last named connectionincluding a Volume control potentiometer device for jointly controllingthe signal output from both amplifying channels.

